Methods, systems, and computer program products for authenticating an online user. Authentication involves sending a code from a server to a user device equipped with a source of illumination and a camera capable of capturing video imagery of the online user. The user device receives the code, modulates the source of illumination in accordance with the code, and captures video imagery of the user while the source of illumination is being modulated according to the code. The captured video imagery of the online user is sent to the server where it is analyzed to detect evidence of changes in illumination that correspond to the code. If good correspondence is found, the user may be authenticated. Similar methods may be applied to other biometric data. Applications of the authentication include identify validation, pseudonym verification, and distinguishing human from non human access attempts.